In holographic data storage digital data are stored by recording the interference pattern produced by the superposition of two coherent laser beams, where one beam, the so-called ‘object beam’, is modulated by a spatial light modulator and carries the information to be recorded. The second beam serves as a reference beam. The interference pattern leads to modifications of specific properties of the storage material, which depend on the local intensity of the interference pattern. Reading of a recorded hologram is performed by illuminating the hologram with the reference beam using the same conditions as during recording. This results in the reconstruction of the recorded object beam.
One advantage of holographic data storage is an increased data capacity. Contrary to conventional optical storage media, the volume of the holographic storage medium is used for storing information, not just a few layers. One further advantage of holographic data storage is the possibility to store multiple data in the same volume, e.g. by changing the angle between the two beams or by using shift multiplexing, etc. Furthermore, instead of storing single bits, data are stored as data pages. Typically a data page consists of a matrix of light-dark-patterns, i.e. a two dimensional binary array or an array of grey values, which code multiple bits. This allows to achieve increased data rates in addition to the increased storage density. The data page is imprinted onto the object beam by the spatial light modulator (SLM) and detected with a detector array.
For any holographic data storage system it is essential to be robust against undesired variations of the system parameters, both during the reading and the writing process. Such variations result of, for example, temperature changes or differing properties of the used components of different systems. Especially variations in the setup of the light source, which typically is a laser diode, the spatial light modulator and the detector need to be controlled and minimized for a well working system. For example, even small changes of the wavelength of the readout reference beam compared to the recording reference beam (also known as wavelength mismatch) lead to significant distortions of the readout data page. Due to the variations the dependency of the detector pixel values on the SLM pixel values may have a different characteristic for each pixel of the detector. As a result the detector image can be non-uniform in image contrast and intensity. This reduced image quality can in turn increase the symbol error rate.
A further aspect that needs consideration is the interchangeability, i.e. using a holographic storage medium with different holographic data storage systems. Interchangeability has to be ensured for a market acceptance of holographic data storage. Slight and maybe unforeseen differences of the properties of the holographic storage media should be compensated by the holographic data storage system.
To cope with the above problems a calibration of a holographic storage system is necessary. For example, a fixed calibration can be determined during the manufacturing process. Alternatively, highly advanced components with negligible tolerances can be used.
In EP 0 201 274 a holographic storage system is described, in which a readout calibration is implemented. During recording calibration pixels with predetermined grey values are recorded on a holographic storage medium. During read-out these reference patterns are evaluated for normalizing the pixel intensity. The calibration pixels are provided for compensating for variations caused by superimposed additional holograms on the storage medium and are recorded in each data page.
The document U.S. Pat. No. 5,920,536 discloses a holographic storage system using page marks for obtaining a quality measure during readout. Based on this quality measure geometrical or optical parameters are varied. The resulting quality is measured as a calibration feedback.
The document WO 2005/103842 discloses a holographic storage medium having calibration features. Special indicators are provided at specified positions on the storage medium. These indicators provide information about the storage medium for adjusting the holographic storage system to the storage medium.